Simplicity Regent EX throws drive belt

Hello all, I have a Simplicity Regent EX lawn tractor. It has 137hrs on it and began throwing the drive belt off of the transmission suddenly. I have replaced the belt and confirmed it was the right belt from the dealer and it continues to lose the belt off of the transmission. I have inspected all pulleys and confirmed I have routed it correctly. No problems found. The belt just seems too loose when the parking brake is engaged. The tractor will move fine but when I stop and put parking brake on with engine running it throws the belt right off. Please help I don't know what else to do. Everything is tight with transmission mounting. Maybe a shorter belt??

Ge to simplicity and download the IPL.
From that locate all of the belt keepers then check that your mower has them all still in place.
Some of the plain bolt types can drop off.

Also check the transmission
Again from the IPL you will see it is held in by 2 bolts either side of the axel housings plus a stabilizer at the front either as a bracket or bolting directly into the frame.
These bolts frett loose , break or drop out allowing the tranny to tip & throw off the belt.